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import Foundation
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/// A MonadError has the same capabilities as a `Monad` and an `ApplicativeError`.
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public protocol MonadError: Monad, ApplicativeError {}
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// MARK: Related functions
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public extension MonadError {
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/// Checks if the value of a computation matches a predicate, raising an error if not.
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///
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/// - Parameters:
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/// - fa: A computation in the context implementing this instance.
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/// - error: A function that produces an error of the type this instance is able to handle.
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/// - predicate: A boolean predicate to test the value of the computation.
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/// - Returns: A value or an error in the context implementing this instance.
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static func ensure<A>(_ fa: Kind<Self, A>, _ error: @escaping () -> E, _ predicate: @escaping (A) -> Bool) -> Kind<Self, A> {
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return flatMap(fa, { a in
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predicate(a) ? pure(a) : raiseError(error())
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})
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}
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}
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// MARK: Syntax for MonadError
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public extension Kind where F: MonadError {
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/// Checks if the value of this computation matches a predicate, raising an error if not.
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///
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/// This is a convenience method to call `MonadError.ensure` as an instance method of this type.
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///
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/// - Parameters:
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/// - error: A function that produces an error of the type this instance is able to handle.
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/// - predicate: A boolean predicate to test the value of the computation.
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/// - Returns: A value or an error in the context implementing this instance.
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func ensure(_ error: @escaping () -> F.E, _ predicate: @escaping (A) -> Bool) -> Kind<F, A> {
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return F.ensure(self, error, predicate)
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}
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}
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truizlop